How does C++ achieve the effect of copying?

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C.61: copy operation should have the effect of copy

Reason (reason)

This is a conventional semantics. When xroomy is executed, we should also think of it as xroomy. After copying the action, x and y can be two independent objects (value semantics, like non-pointer built-in types and standard library types) or different references to the same shared object (pointer semantics, like pointer behavior).

Example (example) class X {/ / OK: value semantics

Public:

X ()

X (const X &); / / copy X

Void modify (); / / change the value of X

/ /...

~ X () {delete [] p;}

Private:

T * p

Int sz

}

Bool operator== (const Xa, const Xb)

{

Return a.sz = = b.sz & & equal (A.P, A.P + a.sz, b.p, b.p + b.sz)

}

XRV X (const Xa)

: p {new T [a.sz]}, sz {a.sz}

{

Copy (A.P, A.P + sz, p)

}

X x

X y = x

If (x! = y) throw Bad {}

X.modify ()

If (x = = y) throw Bad {}; / / assume value semanticsExample (example) class X2 {/ / OK: pointer semantics

Public:

X2 ()

X2 (const X2 &) = default; / / shallow copy

~ X2 () = default

Void modify (); / / change the pointed-to value

/ /...

Private:

T * p

Int sz

}

Bool operator== (const X2 & a, const X2 & b)

{

Return a.sz = = b.sz & & a.p = = b.p

}

X2 x

X2 y = x

If (x! = y) throw Bad {}

X.modify ()

If (x! = y) throw Bad {}; / / assume pointer semanticsNote (note)

Unless you are building some kind of "smart pointer", the value semantics is better. Value semantics are the easiest to understand and are expected by the standard library function.

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